This invention relates to a pulse power supply circuit for lasers, including gas-filled control switching elements, and more particularly to a power supply circuit for pulsed lasers suitable for high pulse repetition rates of over 1 kHz.
In conventional power supply circuits for pulsed lasers, each including a plurality of gas-filled control switching elements connected in parallel, a plurality of parallel gas-filled control switching elements (referred to as thyratrons for short) are arranged to form the plasma at the same time as described in page 22 of the prelease of papers for 1987 Research Results Report Session by the Laser Technology R&D Laboratory and in the prerelease of papers for the 35th Applied Physics Joint Lecture of 1988, 30P-ZL-14. The simultaneous, parallel operating of thyratrons is executed to lighten the load for the thyratrons and prolong their service life. However, since the closing time is on the order of nsec, it is extremely difficult to have the simultaneous closure occur stably for a long time. The gas-filled control switching element has a characteristic that the allowable voltage and the allowable peak voltage decrease as the repetitive operation rate is increased. These allowable upper limits are determined by the product (pb: anode dissipation factor) of the three values: voltage, repetition rate, and peak voltage of the switching element. However, the repetition rate has the upper limit to which the switching element can be used in a suitable manner, and it is technically difficult to use a switching element beyond the maximum allowable repetition rate. Generally, gas-filled control switching elements have the maximum allowable repetition rates, and are used at less than the maximum allowable repetition rate.
It is a trial practice to use a plurality of gas-filled control switching elements connected in parallel and thereby increase the allowable current proportional to the number of elements employed when they are used at or under the maximum allowable repetition rate. The repetition rate of switching elements is determined by the insulation property recovery speed: therefore, if a voltage is applied to the switching element before the insulation property recovers, there is a fear that dielectric breakdown occurs. The insulation property recovery speed is higher for smaller current flowing through the switching element. However, the insulation resistance changes exponentially from several ohms during conduction of the switching element to several hundred megohms during recovery of the insulation property For example, even if the current flowing through the switching element is reduced to, say, 1/10, the insulation property recovery time
In actuality, for gas-filled control switching elements, actual conditions more than this maximum allowable repetition rate has not been determined definitely. There is the difficulty concerning the high repetition rate in this respect.
Generally, for the gas-filled control switching elements, the pb value determines the limit values as mentioned above. The inventor of this patent application has found that the allowable voltage falls notably at repetition rates greater than the maximum allowable repetition rate, and that this voltage hardly of varies with the current value. For example, when the repetition rate was 5 kHz, the allowable voltage was 16 kV and the allowable power was 4 kW. On the other hand, at a repetition rate of 2.5 kHz, the switching element was operable up to an allowable voltage of 21 kV and an allowable power of 5.5 kW. Accordingly, when two gas-filled control switching elements were connected in parallel and operated simultaneously, the allowable voltage was 16 kV and the allowable power was 8 kW at a repetition rate of 5 kHz. In contrast, according to this invention, when switching elements were operated alternately, the switching elements were operable at an allowable voltage of 21 kV and an allowable power of 11 kW at a total repetition rate of 5 kHz.